The invention is directed to an integratable transistor switching circuit of the NTL (non-threshold logic) family.
The fastest standard logic circuits on a silicon substrate currently work in ECL Technology (emitter-coupled-logic). In this technology, the beneficial properties of bipolar transistors are utilized, these being operated with currents and voltages below the saturation limit. A typical feature of this logic family is the connection of two bipolar transistors at their emitters. The base of one of these transistors serves as an input of the basic gate, whereas the base of the other transistor is charged with a constant voltage. The value of this voltage lies between the voltage values defined for the logical statuses of high and low. A reliable switching of the voltage value at the output given defined voltage values at the input of an ECL gate is thus guaranteed. The switching occurs by inhibiting the previously conductive transistor and by activating the previously inhibited transistor. Since, however, both transistors are connected at their emitters, the aggregate current of both emitter current paths is always constant and unequal to zero. In every logical status, thus power is consumed.
A logic family that works just as fast as the ECL Technology but which consumes power in only one of the two possible logical statuses is the logic family in NTL Technology (non-threshold logic). The basic gate of this logic family according to the reference of Rein/Ranfft, "Integrierte Bipolarschaltungen", Springer-Verlag, 1987, page 191, incorporated herein, is formed of a bipolar transistor having a collector and an emitter resistor. A constant supply voltage is present between collector and emitter. The base of the transistor serves as a gate input and the collector serves as a gate output. The voltages present at the transistor are selected such that the transistor--as in ECL Technology--is never saturated. The logical statuses at the gate output are achieved by the activation or inhibiting of the transistor, and thus of the current path. Statistically considered, the power consumption in the NTL Technology is cut in half in comparison to the ECL Technology, this establishing a substantial advantage with respect to power consumption. Two reasons nonetheless oppose an employment of the NTL Technology. No complementary gate outputs as in ECL Technology are present, and the transmission characteristic of the NTL gates does not have a defined switching threshold and does not have a constant output voltage independent of the input voltage in any logical status. A defined switching threshold and constant logical voltage levels, however, are a basic prerequisite for realizing a malfunction-resistant logic circuit.
In order to make logic circuits of the NTL family useful in practice, techniques are therefore undertaken in order to make the negative properties of the NTL circuits controllable. One possible circuit-oriented technique is the variation of the voltage at the emitter side corresponding to the properties of the switching transistor. The transmission characteristic of the transistor is influenced by controlling a variable, but constant, supply voltage such that the curve of the transmission characteristic reaches an optimum. The only thing the obtainable optimum guarantees, however, is that the dependency of the output voltage on the input voltage of a gate can be predetermined, and thus, can be manipulated. A reliability of the switching functions that is comparable to the reliability known from ECL technology, however, cannot be achieved. The method of keeping the once-defined emitter resistance constant but varying the supply voltage at the emitter current path of the bipolar transistor harbors the disadvantage that an involved control circuit is required for the variable but constant supply voltage, since the voltage source must supply a relatively high current to the low-impedance circuit input.